There is much debate on whether continuous exposure of commensal bacteria and potential pathogens residing in the human intestinal tract to low levels of antimicrobial agents from treated food animals pose a public health concern. To investigate antimicrobial effects on bacteria under colonic conditions, we studied resistance development in Salmonella enterica
and Listeria monocytogenes
exposed to enrofloxacin in the presence of fecal extract. The bacteria were incubated at 37 °C in Mueller-Hinton broth, with and without 0.01~0.5 μg/mL enrofloxacin, in the presence and absence of sucrose, and with 1% or 2.5% filter-sterilized fecal extract, for three passages. In the second and third passages, only the bacteria incubated in the media containing sterilized fecal extract grew in 0.5 μg/mL of enrofloxacin. Fecal extract (1% and 2.5%) decreased the sensitivity of S. enterica
to enrofloxacin in the medium containing the efflux pump inhibitors reserpine and carbonyl cyanide-m
-chlorophenylhydrazone (CCCP) and affected the accumulation of ethidium bromide (EtBr) in this bacterium.
Enrofloxacin (0.06 µg/mL) and fecal extract altered the composition of fatty acids in S. enterica
and L. monocytogenes.
We conclude that fecal extract decreased the susceptibilities of S. enterica
and L. monocytogenes
to concentrations of enrofloxacin higher than the MIC and resulted in rapid resistance selection.